investigating audio-drive bug

2 files changed, 15 insertions, 0 deletions

diff --git a/src/audio/audio.cc b/src/audio/audio.cc
index aaadd5a..ba76a28 100644
--- a/src/audio/audio.cc
+++ b/src/audio/audio.cc
@@ -97,6 +97,11 @@ void audio_render_ahead(float music_time, float dt, float target_fill) {
 
   // Render in small chunks to keep synth time synchronized with tracker
   // Chunk size: one frame's worth of audio (~16.6ms @ 60fps)
+  // TODO(timing): CRITICAL BUG - Truncation here may cause 180ms drift over 63 beats
+  // (int) cast loses fractional samples: 0.333 samples/frame * 2560 frames = 853 samples = 27ms
+  // But observed drift is 180ms, so this is not the only source (27ms < 180ms)
+  // NOTE: This is NOT a float vs double precision issue - floats handle <500s times fine
+  // See also: tracker.cc BPM timing calculation
   const int chunk_frames = (int)(dt * RING_BUFFER_SAMPLE_RATE);
   const int chunk_samples = chunk_frames * RING_BUFFER_CHANNELS;
 
diff --git a/src/audio/tracker.cc b/src/audio/tracker.cc
index 1c0a9b2..37f0683 100644
--- a/src/audio/tracker.cc
+++ b/src/audio/tracker.cc
@@ -234,6 +234,16 @@ static void trigger_note_event(const TrackerEvent& event,
 }
 
 void tracker_update(float music_time_sec, float dt_music_sec) {
+  // TODO(timing): CRITICAL BUG - Events trigger ~180ms early over 63 beats @ BPM=90
+  // Observed: Beat 63 snare at 41.82s in WAV, should be at 42.00s (180ms drift)
+  // NOTE: This is NOT a float vs double precision issue - floats handle <500s times fine
+  // Root cause unknown - suspects:
+  //   1. Systematic bias in time calculation (not random accumulation)
+  //   2. Truncation in audio.cc:103 chunk_frames = (int)(dt * sample_rate)
+  //   3. BPM calculation precision below (unit_duration_sec)
+  //   4. Mismatch between tracker time and actual sample rendering
+  // See also: audio.cc sample rendering truncation
+
   // Unit-less timing: 1 unit = 4 beats (by convention)
   const float BEATS_PER_UNIT = 4.0f;
   const float unit_duration_sec =